A typical control valve has a linear motion closure member commonly referred to as a plug (or piston), one or more ports, and a body typically distinguished by a globular shaped cavity around the port region. The body is the part of the valve which is the main pressure boundary. The body typically provides the pipe connecting ends and the fluid flow passageway. In a control valve, the closure member is a movable part of the valve that is positioned in the flow path to modify the rate of flow through the valve.
The plug closure member is a part, often cylindrical in nature, which moves in the flow stream with linear motion to modify the flow rate. It may or may not have a contoured portion to provide flow characterization. It may also be a cylindrical or conically tapered part. In other types of valves, the closure member includes ball, disk and gate.
A flow orifice in the flow passageway (path) interacts with the closure member to close the valve. The orifice may be provided with a seating surface, to be contacted by or closely fitted to the closure member, to provide tight shut-off or limited leakage, i.e., to close the valve.
A cage, guide or liner is typically a part in a control valve that generally surrounds the closure member to provide alignment and facilitate assembly of other parts of the valve trim. The cage may also provide flow characterization and/or a seating surface. Control valve trim typically includes the internal parts of a valve which are in flowing contact with the controlled fluid. Examples of valve trim are the plug, seat ring and cage. The body is not considered part of the trim.
FIG. 1 illustrates a standard prior art control valve 80 manufactured and sold by Dresser, Inc.'s Masoneilan® division. A valve stem 1 extends through a valve bonnet 7 to a valve plug 15. Valve plug 15 is received in a cage 16. A seal 30 is disposed between the plug 15 and the inner wall of cage 16. FIGS. 2A and 2B illustrate different embodiments of the seal 30 between the plug (piston) 15 and the inner wall of cage 16. FIG. 2B shows a two-part seal with an inner member 31 that can be formed of Ni-Resist metal and an outer member 32 that can be formed of structural graphite. In this valve, the seal is located in a seal groove (gland) on the plug. In other variations, the seal may be located in a seal groove on the cage wall.
TABLE 1Seat Leakageper IEC534-4 andValveValve SizeTemperature Range(1)ANSI/FCIModelSeal TypeinchesMmMinimumMaximum70.2 Class41305Pressure2-1650-400 −50° F. +450° F. (+232° C.)IV (standard)Energized (−46° C.)V (optional)PTFE SealRing41405Auxiliary Pilot2-4 50-100−320° F. +850° F. (+454° C.)(2)IV (standard)Plug with(−196° C.)V (optional)Metal Seal6-16150-400 −320° F.+1050° F. (+566° C.)Ring(−196° C.)41505Metal Seal2-4 50-100−320° F. +850° F. (+454° C.)(2)IIRing(−196° C.)6-16150-400 −320° F.+1050° F. (+566° C.) III(−196° C.)41605PTFE Seal2-1650-400 −20° F. +300° F. (+149° C.)IVRing (−29° C.)41905Graphite Seal2-4 50-100−320° F. +850° F. (+454° C.)IVRing(−196° C.)6-16150-400 −320° F. +850° F. (+454° C.)(−199° C.)Notes:(1)See Materials of Construction Tables for other temperature limitations.(2)Maximum temperature limit for the 2° (50 mm) size is +1050° F. (+566° C.)
Table 1 includes information on the manufacturer's suggestions regarding selection of the particular type of seal depending on the temperature range of the fluid flowing through the valve. In high temperature service, the prior art valves use rigid graphite seals. These rigid seals include a split or break in the circumference that allows the seal to conform to the diameter of the sealing surface. Such a split ring seal is similar to a piston ring used to seal an internal combustion engine piston in the engine cylinder. Because the circumference of the seal ring is split, fluid leaks across the seal.
Other prior art high temperature seals may be-formed of compressed flexible graphite. This flexible graphite prior art seal results in a high sliding friction, and a high rate of wear makes it unsuitable for automated positioning/throttling applications of control valves.